A multi-emitter laser diode produces a plurality of optical beams, one from each emitter. A common method of delivering the laser diode optical output to an intended application includes coupling the optical beams into a bundle of transport optical fibers. The input ends of the transport fibers are aligned to the laser diode emitters. Coupling optics are placed between the emitters and the transport fiber input ends to properly focus the laser diode output beams into the array of transport fibers. It is known to place the laser diode, coupling optics and transport fiber input ends inside a sealed enclosure to prevent contamination of the laser diode emitters, coupling optics and transport fiber input ends. Since the laser diode generates heat that must be efficiently conducted out of the enclosure during operation, copper is the material of choice for such laser diode enclosures. The copper material provides an efficient thermal conduction path out of the laser diode enclosure.
Sealed laser diode enclosures are typically fabricated from a single, heavily-machined, copper block. The block is hollowed out to form a base portion and wall portions extending upwards and terminating at a top opening. The laser diode, coupling optics and transport fibers are mounted inside the hollowed enclosure. Heat generated by the diode is conducted out of the enclosure through the base portion. Sealed ports are formed in the side walls of the enclosure to allow electrical and optical signals to penetrate the enclosure. A lid is glued over the top opening to hermetically seal the enclosure.
There are several disadvantages in forming the laser diode enclosure from a single block of material, especially copper. First, it is complicated and expensive to machine the enclosure out a single block of material. This process also wastes the material removed from the block to form the enclosure. In addition, the high thermal conductivity of the copper walls (about 388 W/m.multidot.K) makes it difficult or impossible to use desirable high temperature metallurgical bond forming techniques, such as soldering or welding, to attach the lid onto the side walls. Instead, it is common to glue the lid onto the copper walls of the enclosure. Glue, however, is difficult and messy to apply, a glue bond is not as repeatable or reliable as metallurgical bonds, glue takes time to cure, and glue can outgas during and/or after curing which can contaminate the interior of the enclosure.
It is conceivable to use a less thermally conductive enclosure material to facilitate soldering or welding of the lid to the enclosure walls. However, the base portion would not adequately conduct the thermal output of the laser diode out of the enclosure as required during laser diode operation.
One solution has been to fabricate the laser diode enclosure with a copper base plate and four Kovar metal side walls attached together and attached to the copper base. A Kovar metal lid is attached to the top of the Kovar side walls. While Kovar metal has a relatively low thermal conductivity (about 17 W/m.multidot.K), this configuration has several drawbacks. First, Kovar has a much lower coefficient of thermal expansion (about 5 ppm/.degree. C.) than copper (about 18 ppm/.degree. C.). Therefore, it is difficult to provide a quality direct metallurgical bond between the Kovar side walls and the copper base plate, either by soldering, brazing or welding. A good metallurgical bond between the base plate and walls is preferred because it results in a good strong seal while providing good thermal and electrical conductivity. The difference in expansion of the Kovar walls relative to the copper base plate during a soldering, welding, and/or brazing operation is too great to allow these processes to yield a reliable high quality metallurgical bond. Therefore, copper skirts and other expensive and complicated attachment assemblies and techniques have instead been used to connect Kovar side walls to copper base plates.
There is a need for simple, inexpensive laser diode enclosure that facilitates easy and reliable attachment of the side walls to the base plate and the lid with high quality metallurgical bonds. Such an enclosure should hermetically seal the laser diode and delivery system optics for contamination protection, while providing an efficient and reliable heat path to conduct the thermal load of the laser diode out of the enclosure.